The processing industry relies upon the accurate measurement of process variables to enable efficient control of industrial processes. One of the most common methods of taking various process measurements is via process tapping points exposed to the interior contents of a process vessel or pipeline. Flow and/or pressure through an internal bore of the tapping points is indicative of certain process variables.
Such tapping points encounter progressive scaling or debris build-up over time within their internal bores. The restriction or blockage of process tapping points by scaling or debris build-up can cause inaccurate process measurement, inaccurate product sampling or even render the process measurement completely unavailable.
Therefore, when process tapping points are blocked or restricted to the point of affecting the accuracy of the process measurement, they need to be cleared.
Traditional methods used in the clearance of process tapping points include the removal of the blocking material using manual or power tools whilst the process is online. Given the toxic and/or flammable nature of most process fluids, this can be a highly hazardous operation for even the most experienced operators.
Another method used to maintain the clearance of the process tapping points is the introduction of a purge fluid through the tapping points. This purge fluid passes continuously through the tapping point, delaying the settling of scaling or debris. The disadvantage of this method is that it adds a large amount of inert media to the process fluid that must later be extracted at significant expense to maintain process efficiency. Further, the introduction of purge fluid through the tapping point does not stop the tapping point blocking, but merely delays the blocking.
Similarly, another way of slowing blockage is to provide larger diameter tapping points so that the time taken to block is longer. Large, oversize process connections have been utilised to provide for longer periods where accurate process measurements can be obtained. This arrangement merely delays the inevitable need to clear the process tapping points. Oversize process connections are also more expensive to install than conventional connections.
International application publication number WO02/36276, the contents of which are incorporated herein by reference, discloses an automated tapping point clearing apparatus. The apparatus includes a clearing tool employed on a reciprocating shaft. The clearing tool includes a scraping ring arranged to clear debris built up on an internal bore of a tapping point. The clearing tool also includes a plurality of apertures to allow fluids to flow through the clearing tool during a cleaning operation thereby maintaining accurate process variable measurement even during the passage of the clearing tool through the aperture.
Tapping point clearing apparatuses made in accordance with that international application have proved useful and reliable. Nonetheless, certain deficiencies in their use have become apparent.
In use, during an advance (clearing tool extending) or retract (clearing tool returning) stroke, the clearing tool on the end of the shaft, and the piston and its attached shaft within the cylinder of the clearing tool, are exposed to rapid acceleration and deceleration forces during extension and retraction phases. The shaft is operable by a pneumatic piston, and it is imperative that the clearing tool is advanced rapidly in order to “punch through” any potential build up of process material covering the tapping point. However, the piston (with attached shaft) within the cylinder has to be stopped rapidly at the end of the advance and retract strokes. A known arrangement for controlling the rapid deceleration forces at the end of each stroke is to provide resilient limit of travel stops, such as rubber or rubberised stops. The piston hits the limit of travel stop in either direction, thereby providing a cushioned stop for the piston.
However, despite such impact cushioning, the piston and attached shaft undergo rapid deceleration over a very short distance. This puts severe stress on the joint between the shaft and piston. Typically the shaft has a threaded piston end machined down to a reduced diameter in order to provide a shoulder at that end of the shaft on the un-machined portion. That shoulder provides a seat and thread limit against the face of the piston. The thread on the end of the shaft is received in a corresponding threaded blind aperture in the centre of the piston. Consequently, when the piston and shaft undergo rapid deceleration at full extension, the reduced diameter shaft adjacent the full diameter shoulder at the shoulder can fail. This can result in the clearing tool and at least a portion of the piston being lost into the process vessel or pipe. Understandably, such a failure can result in damage to process equipment and machinery, such as blockage of valves or pumps downstream of the failed tapping point clearing apparatus. Furthermore, the tapping point clearing apparatus has to be repaired or replaced. Either way, the process may be shut down until such time as the clearing apparatus is repaired and the lost shaft recovered. This results in economically expensive downtime for the process plant as well as risk to personnel having to recover the lost shaft from a toxic, extremely hot or corrosive process stream.
A further deficiency in the operation of the apparatus of WO02/36276 becomes when providing an external visual indication as to the position of the clearing tool, in order to identify errors in operation of the tapping point clearing apparatus. A known previous apparatus achieved this by use of a clear plastic tube fixed to the external surface of the ram and containing coloured magnets that travelled the full length of the clear tube directly adjacent to a magnet fixed to the piston inside the ram. This visual indication was found to work effectively only in installations where the complete unit could be installed horizontally. In vertical or inclined installations gravity prevented continual position indication as the coloured magnet inside the clear tube would not reliably slide upwards. As it got covered in dust, we then could not see the orange magnet, as it got older the friction inside the clear plastic tube meant that sometimes the orange magnet sticks or does not follow the magnet attached to the piston because of ingress of residue or dirt into the tube. Also, the clear tube housing the coloured magnet would discolour over time, rendering the moving magnet even more difficult to observe even if it did move.
The present invention has been created in light of these practical deficiencies in the operation of the prior art.
Although described with reference to process industry it would be clear to a person skilled in the art that the present invention has applicability to a number of industries where access is required to a pipe or vessel that scales during use.